1. Field of the Invention
The present invention relates to an evaporation source having a nozzle structure that is suitable for continuously forming a thin film on a deposited substrate by vapor deposition and a vacuum evaporator using the same.
2. Description of the Related Art
As is well-known, a vacuum evaporator causes a thin film to be formed by arranging an evaporation material and a deposited substrate in a vacuum chamber, melting the evaporation material by heating to cause vaporization through evaporation or sublimation, and causing the vaporized evaporation material to be deposited on the surface of a deposited substrate. Heater heating, high-frequency heating, beam heating and the like have been proposed as heating methods of the evaporation material in the above-mentioned vacuum evaporator and, though each heating method has its own special features, generally an external heating crucible method by which a vaporizing chamber (crucible) housing an evaporation material is heated by an external heater is frequently used.
Then, in recent years, by using a vacuum evaporator, vapor deposition is not limited to metallic materials, and formation of organic thin films by vapor deposition of organic substance and thin films by vapor code position using a plurality of organic substances is also performed. Accordingly, various forms, for example, a flat panel display (FPD) using an organic electroluminescence device and illumination by organic electroluminescence device used as a surface light source (luminescent panel) have been proposed.
The substrate used in the FPD and that for the luminescent panel used for illumination and the like are becoming increasingly larger in size and, in response to such trends, a vacuum evaporator using a linear evaporation source that causes a deposited substrate to be transferred in one direction and on which a long nozzle is formed in a direction perpendicular to the transfer direction of the substrate, that is, the width direction of the substrate has been provided. Accordingly, a vapor can be applied in the width direction of the substrate while the deposited substrate is transferred in one direction and therefore, an evaporated film can continuously be formed on a substrate having a relatively large area.
When the above-mentioned linear evaporation source is used to evaporate, for example, an organic material onto a substrate, it is important that the thickness of film to be evaporated is uniformly formed. However, in a vacuum evaporator using the above-mentioned linear evaporation source, it becomes difficult to form a uniform density distribution of vaporized evaporation material and an ideal flow of vapor as the whole cabinet including the vaporizing chamber housing an evaporation material becomes longer with an ever larger deposited substrate. Accordingly, the amount of evaporation on the substrate surface becomes uneven, making it difficult to obtain a uniform distribution of film thickness particularly in the width direction of the substrate.
FIG. 13 schematically shows an example of a vacuum evaporator using the above-mentioned linear evaporation source. Reference numeral 1 shows a linear evaporation source and its contour is constituted by a cabinet part 1A formed approximately like a long rectangular parallelepiped. Then, a top end of the cabinet part 1A has a rectangular opening, thereby forming a nozzle opening 2.
Though not shown in FIG. 13, a lower base part of the cabinet part 1A is similarly formed in a long shape to constitute a vaporizing chamber (crucible) in which an evaporation material is housed. Then, the whole cabinet part 1A including the vaporizing chamber is constituted in such a way that the cabinet part 1A is heated by an external heater (not shown). With this constitution, the evaporation material housed in the vaporizing chamber is vaporized or sublimated by heating and a vapor thereof is discharged like a band in a vertical direction from the nozzle opening 2.
Immediately above the nozzle opening 2, on the other hand, a deposited substrate 3 is constituted at a distance H from the upper face of the nozzle opening 2 so that the deposited substrate 3 is transferred at a constant speed in a direction indicated by an arrow A. That is, the nozzle opening 2 is arranged in such a way that its longitudinal direction is perpendicular to the transfer direction of the substrate 3, thereby enabling application of a vapor linearly (like a band) in the width direction of the substrate 3. Therefore, by transferring the deposited substrate 3 in the direction of the arrow A, evaporation can be performed continuously on the substrate 3 having a relatively large area.
A vacuum evaporator that uses a linear evaporation source as described above and performs evaporation continuously onto a deposited substrate being transferred at a constant speed has been disclosed in Japanese Patent Application Laid-Open No. 8-27568 (Patent Document 1) and Japanese Patent Application Laid-Open No. 2003-293120 (Patent Document 2).
Meanwhile, a linear evaporation source shown in the above-mentioned Patent Documents 1 and 2 embodies some constitutional devices on the assumption that it is desirable to make the flow rate of evaporated material constant (uniform) at any position in the longitudinal direction of the nozzle opening in the evaporation source in order to make the thickness of film formed on a deposited substrate uniform.
However, the present inventors have empirically found that if the distribution of flow rate of evaporated material in the longitudinal direction of the nozzle opening is made uniform, a phenomenon occurs in which the thickness of film formed on a substrate by vapor deposition becomes thinner along the width direction of the substrate toward an edge thereof. FIG. 14 illustrates a film thickness distribution thereof, and the horizontal axis shows the position in the width direction of the substrate 3 with a center C and the vertical axis shows the ratio of film thickness when the thickness of film formed in the center of the substrate 3 is set to 100.
The thickness of film formed on the basis of an idea to make the distribution of flow rate of evaporated material in the longitudinal direction of the nozzle opening uniform in the above-mentioned linear evaporation source results in, as shown in FIG. 14, a thinner edge in the width direction of a substrate. Therefore, a range (area) B1 in which the thickness of film is formed uniformly on the substrate will be narrow compared with a dimension in the width direction of the substrate.